Request For Comments - RFC4192

Network Working Group F. Baker
Request for Comments: 4192 Cisco Systems
Updates: 2072 E. Lear
Category: Informational Cisco Systems GmbH
R. Droms
Cisco Systems
September 2005
Procedures for Renumbering an IPv6 Network without a Flag Day
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes a procedure that can be used to renumber a
network from one prefix to another. It uses IPv6's intrinsic ability
to assign multiple addresses to a network interface to provide
continuity of network service through a "make-before-break"
transition, as well as addresses naming and configuration management
issues. It also uses other IPv6 features to minimize the effort and
time required to complete the transition from the old prefix to the
new prefix.
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Table of Contents
1. Introduction ....................................................21.1. Summary of the Renumbering Procedure .......................31.2. Terminology ................................................41.3. Summary of What Must Be Changed ............................41.4. Multihoming Issues .........................................52. Detailed Review of Procedure ....................................52.1. Initial Condition: Stable Using the Old Prefix .............62.2. Preparation for the Renumbering Process ....................62.2.1. Domain Name Service .................................72.2.2. Mechanisms for Address Assignment to Interfaces .....72.3. Configuring Network Elements for the New Prefix ............82.4. Adding New Host Addresses ..................................92.5. Stable Use of Either Prefix ...............................102.6. Transition from Use of the Old Prefix to the New Prefix ...102.6.1. Transition of DNS Service to the New Prefix ........102.6.2. Transition to Use of New Addresses .................102.7. Removing the Old Prefix ...................................112.8. Final Condition: Stable Using the New Prefix ..............113. How to Avoid Shooting Yourself in the Foot .....................123.1. Applications Affected by Renumbering ......................123.2. Renumbering Switch and Router Interfaces ..................123.3. Ingress Filtering .........................................133.4. Link Flaps in BGP Routing .................................134. Call to Action for the IETF ....................................144.1. Dynamic Updates to DNS Across Administrative Domains ......144.2. Management of the Reverse Zone ............................145. Security Considerations ........................................146.Acknowledgements ...............................................167. References .....................................................177.1. Normative References ......................................177.2. Informative References ....................................17Appendix A. Managing Latency in the DNS ..........................201. Introduction
The Prussian military theorist Carl von Clausewitz [Clausewitz]
wrote, "Everything is very simple in war, but the simplest thing is
difficult. These difficulties accumulate and produce a friction,
which no man can imagine exactly who has not seen war.... So in war,
through the influence of an 'infinity of petty circumstances' which
cannot properly be described on paper, things disappoint us and we
fall short of the mark". Operating a network is aptly compared to
conducting a war. The difference is that the opponent has the futile
expectation that homo ignoramus will behave intelligently.
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A "flag day" is a procedure in which the network, or a part of it, is
changed during a planned outage, or suddenly, causing an outage while
the network recovers. Avoiding outages requires the network to be
modified using what in mobility might be called a "make before break"
procedure: the network is enabled to use a new prefix while the old
one is still operational, operation is switched to that prefix, and
then the old one is taken down.
This document addresses the key procedural issues in renumbering an
IPv6 [RFC2460] network without a "flag day". The procedure is
straightforward to describe, but operationally can be difficult to
automate or execute due to issues of statically configured network
state, which one might aptly describe as "an infinity of petty
circumstances". As a result, in certain areas, this procedure is
necessarily incomplete, as network environments vary widely and no
one solution fits all. It points out a few of many areas where there
are multiple approaches. This document updates [RFC2072]. This
document also contains recommendations for application design and
network management, which, if taken seriously, may avoid or minimize
the impact of the issues.
1.1. Summary of the Renumbering Procedure
By "renumbering a network", we mean replacing the use of an existing
(or "old") prefix throughout a network with a new prefix. Usually,
both prefixes will be the same length. The procedures described in
this document are, for the most part, equally applicable if the two
prefixes are not the same length. During renumbering, sub-prefixes
(or "link prefixes") from the old prefix, which have been assigned to
links throughout the network, will be replaced by link prefixes from
the new prefix. Interfaces on systems throughout the network will be
configured with IPv6 addresses from the link prefixes of the new
prefix, and any addresses from the old prefix in services like DNS
[RFC1034][RFC1035] or configured into switches and routers and
applications will be replaced by the appropriate addresses from the
new prefix.
The renumbering procedure described in this document can be applied
to part of a network as well as to an organization's entire network.
In the case of a large organization, it may be advantageous to treat
the network as a collection of smaller networks. Renumbering each of
the smaller networks separately will make the process more
manageable. The process described in this document is generally
applicable to any network, whether it is an entire organization
network or part of a larger network.
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1.2. Terminology
DDNS: Dynamic DNS [RFC2136][RFC3007] updates can be secured through
the use of SIG(0) [RFC4033][RFC4034][RFC4035][RFC2931] and TSIG
[RFC2845].
DHCP prefix delegation: An extension to DHCP [RFC3315] to automate
the assignment of a prefix, for example, from an ISP to a customer
[RFC3633].
flag day: A transition that involves a planned service outage.
ingress/egress filters: Filters applied to a router interface
connected to an external organization, such as an ISP, to exclude
traffic with inappropriate IPv6 addresses.
link prefix: A prefix, usually a /64 [RFC3177], assigned to a link.
SLAC: StateLess Address AutoConfiguration [RFC2462].
1.3. Summary of What Must Be Changed
Addresses from the old prefix that are affected by renumbering will
appear in a wide variety of places in the components in the
renumbered network. The following list gives some of the places that
may include prefixes or addresses that are affected by renumbering,
and gives some guidance about how the work required during
renumbering might be minimized:
o Link prefixes assigned to links. Each link in the network must be
assigned a link prefix from the new prefix.
o IPv6 addresses assigned to interfaces on switches and routers.
These addresses are typically assigned manually, as part of
configuring switches and routers.
o Routing information propagated by switches and routers.
o Link prefixes advertised by switches and routers [RFC2461].
o Ingress/egress filters.
o ACLs and other embedded addresses on switches and routers.
o IPv6 addresses assigned to interfaces on hosts. Use of StateLess
Address Autoconfiguration (SLAC) [RFC2462] or DHCP [RFC3315] can
mitigate the impact of renumbering the interfaces on hosts.
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o DNS entries. New AAAA and PTR records are added and old ones
removed in several phases to reflect the change of prefix.
Caching times are adjusted accordingly during these phases.
o IPv6 addresses and other configuration information provided by
DHCP.
o IPv6 addresses embedded in configuration files, applications, and
elsewhere. Finding everything that must be updated and automating
the process may require significant effort, which is discussed in
more detail in Section 3. This process must be tailored to the
needs of each network.
1.4. Multihoming Issues
In addition to the considerations presented, the operational matters
of multihoming may need to be addressed. Networks are generally
renumbered for one of three reasons: the network itself is changing
its addressing policy and must renumber to implement the new policy
(for example, a company has been acquired and is changing addresses
to those used by its new owner), an upstream provider has changed its
prefixes and its customers are forced to do so at the same time, or a
company is changing providers and must perforce use addresses
assigned by the new provider. The third case is common.
When a company changes providers, it is common to institute an
overlap period, during which it is served by both providers. By
definition, the company is multihomed during such a period. Although
this document is not about multihoming per se, problems can arise as
a result of ingress filtering policies applied by the upstream
provider or one of its upstream providers, so the user of this
document also needs to be cognizant of these issues. This is
discussed in detail, and approaches to dealing with it are described,
in [RFC2827] and [RFC3704].
2. Detailed Review of Procedure
During the renumbering process, the network transitions through eight
states. In the initial state, the network uses just the prefix that
is to be replaced during the renumbering process. At the end of the
process, the old prefix has been entirely replaced by the new prefix,
and the network is using just the new prefix. To avoid a flag day
transition, the new prefix is deployed first and the network reaches
an intermediate state in which either prefix can be used. In this
state, individual hosts can make the transition to using the new
prefix as appropriate to avoid disruption of applications. Once all
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of the hosts have made the transition to the new prefix, the network
is reconfigured so that the old prefix is no longer used in the
network.
In this discussion, we assume that an entire prefix is being replaced
with another entire prefix. It may be that only part of a prefix is
being changed, or that more than one prefix is being changed to a
single joined prefix. In such cases, the basic principles apply, but
will need to be modified to address the exact situation. This
procedure should be seen as a skeleton of a more detailed procedure
that has been tailored to a specific environment. Put simply, season
to taste.
2.1. Initial Condition: Stable Using the Old Prefix
Initially, the network is using an old prefix in routing, device
interface addresses, filtering, firewalls, and other systems. This
is a stable configuration.
2.2. Preparation for the Renumbering Process
The first step is to obtain the new prefix and new reverse zone from
the delegating authority. These delegations are performed using
established procedures, from either an internal or external
delegating authority.
Before any devices are reconfigured as a result of the renumbering
event, each link in the network must be assigned a sub-prefix from
the new prefix. While this assigned link prefix does not explicitly
appear in the configuration of any specific switch, router, or host,
the network administrator performing the renumbering procedure must
make these link prefix assignments prior to beginning the procedure
to guide the configuration of switches and routers, assignment of
addresses to interfaces, and modifications to network services such
as DNS and DHCP.
Prior to renumbering, various processes will need to be reconfigured
to confirm bindings between names and addresses more frequently. In
normal operation, DNS name translations and DHCP bindings are often
given relatively long lifetimes to limit server load. In order to
reduce transition time from old to new prefix, it may be necessary to
reduce the time to live (TTL) associated with DNS records and
increase the frequency with which DHCP clients contact the DHCP
server. At the same time, a procedure must be developed through
which other configuration parameters will be updated during the
transition period when both prefixes are available.
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2.2.1. Domain Name Service
During the renumbering process, the DNS database must be updated to
add information about addresses assigned to interfaces from the new
prefix and to remove addresses assigned to interfaces from the old
prefix. The changes to the DNS must be coordinated with the changes
to the addresses assigned to interfaces.
Changes to the information in the DNS have to propagate from the
server at which the change was made to the resolvers where the
information is used. The speed of this propagation is controlled by
the TTL for DNS records and the frequency of updates from primary to
secondary servers.
The latency in propagating changes in the DNS can be managed through
the TTL assigned to individual DNS records and through the timing of
updates from primary to secondary servers. Appendix A gives an
analysis of the factors controlling the propagation delays in the
DNS.
The suggestions for reducing the delay in the transition to new IPv6
addresses applies when the DNS service can be given prior notice
about a renumbering event. However, the DNS service for a host may
be in a different administrative domain than the network to which the
host is attached. For example, a device from organization A that
roams to a network belonging to organization B, but the device's DNS
A record is still managed by organization A, where the DNS service
won't be given advance notice of a renumbering event in organization
B.
One strategy for updating the DNS is to allow each system to manage
its own DNS information through Dynamic DNS (DDNS)
[RFC2136][RFC3007]. Authentication of these DDNS updates is strongly
recommended and can be accomplished through TSIG and SIG(0). Both
TSIG and SIG(0) require configuration and distribution of keys to
hosts and name servers in advance of the renumbering event.
2.2.2. Mechanisms for Address Assignment to Interfaces
IPv6 addresses may be assigned through SLAC, DHCP, and manual
processes. If DHCP is used for IPv6 address assignment, there may be
some delay in the assignment of IPv6 addresses from the new prefix
because hosts using DHCP only contact the server periodically to
extend the lifetimes on assigned addresses. This delay can be
reduced in two ways:
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o Prior to the renumbering event, the T1 parameter (which controls
the time at which a host using DHCP contacts the server) may be
reduced.
o The DHCP Reconfigure message may also be sent from the server to
the hosts to trigger the hosts to contact the server immediately.
2.3. Configuring Network Elements for the New Prefix
In this step, switches and routers and services are prepared for the
new prefix but the new prefix is not used for any datagram
forwarding. Throughout this step, the new prefix is added to the
network infrastructure in parallel with (and without interfering
with) the old prefix. For example, addresses assigned from the new
prefix are configured in addition to any addresses from the old
prefix assigned to interfaces on the switches and routers. Changes
to the routing infrastructure for the new prefix are added in
parallel with the old prefix so that forwarding for both prefixes
operates in parallel. At the end of this step, the network is still
running on the old prefix but is ready to begin using the new prefix.
The new prefix is added to the routing infrastructure, firewall
filters, ingress/egress filters, and other forwarding and filtering
functions. Routes for the new link prefixes may be injected by
routing protocols into the routing subsystem, but the router
advertisements should not cause hosts to perform SLAC on the new link
prefixes; in particular the "autonomous address-configuration" flag
[RFC2461] should not be set in the advertisements for the new link
prefixes. The reason hosts should not be forming addresses at this
point is that routing to the new addresses may not yet be stable.
The details of this step will depend on the specific architecture of
the network being renumbered and the capabilities of the components
that make up the network infrastructure. The effort required to
complete this step may be mitigated by the use of DNS, DHCP prefix
delegation [RFC3633], and other automated configuration tools.
While the new prefix is being added, it will of necessity not be
working everywhere in the network, and unless properly protected by
some means such as ingress and egress access lists, the network may
be attacked through the new prefix in those places where it is
operational.
Once the new prefix has been added to the network infrastructure,
access-lists, route-maps, and other network configuration options
that use IP addresses should be checked to ensure that hosts and
services that use the new prefix will behave as they did with the old
one. Name services other than DNS and other services that provide
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information that will be affected by renumbering must be updated in
such a way as to avoid responding with stale information. There are
several useful approaches to identify and augment configurations:
o Develop a mapping from each network and address derived from the
old prefix to each network and address derived from the new
prefix. Tools such as the UNIX "sed" or "perl" utilities are
useful to then find and augment access-lists, route-maps, and the
like.
o A similar approach involves the use of such mechanisms as DHCP
prefix delegation to abstract networks and addresses.
Switches and routers or manually configured hosts that have IPv6
addresses assigned from the new prefix may be used at this point to
test the network infrastructure.
Advertisement of the prefix outside its network is the last thing to
be configured during this phase. One wants to have all of one's
defenses in place before advertising the prefix, if only because the
prefix may come under immediate attack.
At the end of this phase, routing, access control, and other network
services should work interchangeably for both old and new prefixes.
2.4. Adding New Host Addresses
Once the network infrastructure for the new prefix is in place and
tested, IPv6 addresses from the new prefix may be assigned to host
interfaces while the addresses from the old prefix are retained on
those interfaces. The new IPv6 addresses may be assigned through
SLAC, DHCP, and manual processes. If SLAC is used in the network,
the switches and routers are configured to indicate that hosts should
use SLAC to assign IPv6 addresses from the new prefix. If DHCP is
used for IPv6 address assignment, the DHCP service is configured to
assign addresses from both prefixes to hosts. The addresses from the
new prefixes will not be used until they are inserted into the DNS.
Once the new IPv6 addresses have been assigned to the host
interfaces, both the forward and reverse maps within DNS should be
updated for the new addresses, either through automated or manual
means. In particular, some clients may be able to update their
forward maps through DDNS, but automating the update of the reverse
zone may be more difficult as discussed in Section 4.2.
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2.5. Stable Use of Either Prefix
Once the network has been configured with the new prefix and has had
sufficient time to stabilize, it becomes a stable platform with two
addresses configured on each and every infrastructure component
interface (apart from interfaces that use only the link-local
address), and two non-link-local addresses are available for the use
of any host, one in the old prefix and one in the new. This is a
stable configuration.
2.6. Transition from Use of the Old Prefix to the New Prefix
When the new prefix has been fully integrated into the network
infrastructure and has been tested for stable operation, hosts,
switches, and routers can begin using the new prefix. Once the
transition has completed, the old prefix will not be in use in the
network.
2.6.1. Transition of DNS Service to the New Prefix
The DNS service is configured to use the new prefix by removing any
IPv6 addresses from the old prefix from the DNS server configuration.
External references to the DNS servers, such as in the DNS service
from which this DNS domain was delegated, are updated to use the IPv6
addresses from the new prefix.
2.6.2. Transition to Use of New Addresses
When both prefixes are usable in the network, each host can make the
transition from using the old prefix to the new prefix at a time that
is appropriate for the applications on the host. If the host
transitions are randomized, DNS dynamic update mechanisms can better
scale to accommodate the changes to the DNS.
As services become available through addresses from the new prefix,
references to the hosts providing those services are updated to use
the new prefix. Addresses obtained through DNS will be automatically
updated when the DNS names are resolved. Addresses may also be
obtained through DHCP and will be updated as hosts contact DHCP
servers. Addresses that are otherwise configured must be updated
appropriately.
It may be necessary to provide users with tools or other explicit
procedures to complete the transition from the use of the old prefix
to the new prefix, because some applications and operating system
functions may be configured in ways that do not use DNS at all or
will not use DNS to resolve a domain name to a new address once the
new prefix is available. For example, a device that only uses DNS to
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resolve the name of an NTP server when the device is initialized will
not obtain the address from the new prefix for that server at this
point in the renumbering process.
This last point warrants repeating (in a slightly different form).
Applications may cache addressing information in different ways, for
varying lengths of time. They may cache this information in memory,
on a file system, or in a database. Only after careful observation
and consideration of one's environment should one conclude that a
prefix is no longer in use. For more information on this issue, see
[DNSOP].
The transition of critical services such as DNS, DHCP, NTP [RFC1305],
and important business services should be managed and tested
carefully to avoid service outages. Each host should take reasonable
precautions prior to changing to the use of the new prefix to
minimize the chance of broken connections. For example, utilities
such as netstat and network analyzers can be used to determine if any
existing connections to the host are still using the address from the
old prefix for that host.
Link prefixes from the old prefix in router advertisements and
addresses from the old prefix provided through DHCP should have their
preferred lifetimes set to zero at this point, so that hosts will not
use the old prefixes for new communications.
2.7. Removing the Old Prefix
Once all sessions are deemed to have completed, there will be no
dependence on the old prefix. It may be removed from the
configuration of the routing system and from any static
configurations that depend on it. If any configuration has been
created based on DNS information, the configuration should be
refreshed after the old prefixes have been removed from the DNS.
During this phase, the old prefix may be reclaimed by the provider or
Regional Internet Registry that granted it, and addresses within that
prefix are removed from the DNS.
In addition, DNS reverse maps for the old prefix may be removed from
the primary name server and the zone delegation may be removed from
the parent zone. Any DNS, DHCP, or SLAC timers that were changed
should be reset to their original values (most notably the DNS
forward map TTL).
2.8. Final Condition: Stable Using the New Prefix
This is equivalent to the first state, but using the new prefix.
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3. How to Avoid Shooting Yourself in the Foot
The difficult operational issues in Section 2.3, Section 2.6, and
Section 2.7 are in dealing with the configurations of routers and
hosts that are not under the control of the network administrator or
are manually configured. Examples of such devices include Voice over
IP (VoIP) telephones with static configuration of boot or name
servers, dedicated devices used in manufacturing that are configured
with the IP addresses for specific services, the boot servers of
routers and switches, etc.
3.1. Applications Affected by Renumbering
Applications may inadvertently ignore DNS caching semantics
associated with IP addresses obtained through DNS resolution. The
result is that a long-lived application may continue to use a stale
IP address beyond the time at which the TTL for that address has
expired, even if the DNS is updated with new addresses during a
renumbering event.
For example, many existing applications make use of standard POSIX
functions such as getaddrinfo(), which do not preserve DNS caching
semantics. If the application caches the response or for whatever
reason actually records the response on disk, the application will
have no way to know when the TTL for the response has expired. Any
application that requires repeated use of an IP address should either
not cache the result or make use of an appropriate function that also
conveys the TTL of the record (e.g., getrrsetbyname()).
Application designers, equipment vendors, and the Open Source
community should take note. There is an opportunity to serve their
customers well in this area, and network operators should either
develop or purchase appropriate tools.
3.2. Renumbering Switch and Router Interfaces
The configuration and operation of switches and routers are often
designed to use static configuration with IP addresses or to resolve
domain names only once and use the resulting IP addresses until the
element is restarted. These static configurations complicate the
process of renumbering, requiring administration of all of the static
information and manual configuration during a renumbering event.
Because switches and routers are usually single-purpose devices, the
user interface and operating functions (software and hardware) are
often better integrated than independent services running on a server
platform. Thus, it is likely that switch vendors and router vendors
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can design and implement consistent support for renumbering across
all of the functions of switches and routers.
To better support renumbering, switches and routers should use domain
names for configuration wherever appropriate, and they should resolve
those names using the DNS when the lifetime on the name expires.
3.3. Ingress Filtering
An important consideration in Section 2.3, in the case where the
network being renumbered is connected to an external provider, is the
network's ingress filtering policy and its provider's ingress
filtering policy. Both the network firewall's ingress filter and the
provider's ingress filter on the access link to the network should be
configured to prevent attacks that use source address spoofing.
Ingress filtering is considered in detail in "Ingress Filtering for
Multihomed Networks" [RFC3704].
3.4. Link Flaps in BGP Routing
A subtle case arises during step 2 in BGP routing when renumbering
the address(es) used to name the BGP routers. Two practices are
common: one is to identify a BGP router by a stable address such as a
loopback address; another is to use the interface address facing the
BGP peer. In each case, when adding a new prefix, a certain
ambiguity is added: the systems must choose between the addresses,
and depending on how they choose, different events can happen.
o If the existing address remains in use until removed, then this is
minimized to a routing flap on that event.
o If both systems decide to use the address in the new prefix
simultaneously, the link flap may occur earlier in the process,
and if this is being done automatically (such as via the router
renumbering protocol), it may result in route flaps throughout the
network.
o If the two systems choose differently (one uses the old address
and one uses the new address), a stable routing outage occurs.
This is not addressed by proposals such as [IDR-RESTART], as it
changes the "name" of the system, making the matter not one of a flap
in an existing relationship but (from BGP's perspective) the
replacement of one routing neighbor with another. Ideally, one
should bring up the new BGP connection for the new address while the
old remains stable and in use, and only then take down the old. In
this manner, while there is a TCP connection flap, routing remains
stable.
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4. Call to Action for the IETF
The more automated one can make the renumbering process, the better
for everyone. Sadly, there are several mechanisms that either have
not been automated or have not been automated consistently across
platforms.
4.1. Dynamic Updates to DNS Across Administrative Domains
The configuration files for a DNS server (such as named.conf) will
contain addresses that must be reconfigured manually during a
renumbering event. There is currently no easy way to automate the
update of these addresses, as the updates require both complex trust
relationships and automation to verify them. For instance, a reverse
zone is delegated by an upstream ISP, but there is currently no
mechanism to note additional delegations.
4.2. Management of the Reverse Zone
In networks where hosts obtain IPv6 addresses through SLAC, updates
of reverse zone are problematic because of lack of trust relationship
between administrative domain owning the prefix and the host
assigning the low 64 bits using SLAC. For example, suppose a host,
H, from organization A is connected to a network owned by
organization B. When H obtains a new address during a renumbering
event through SLAC, H will need to update its reverse entry in the
DNS through a DNS server from B that owns the reverse zone for the
new address. For H to update its reverse entry, the DNS server from
B must accept a DDNS request from H, requiring that an inter-
administrative domain trust relationship exist between H and B. The
IETF should develop a BCP recommendation for addressing this problem.
5. Security Considerations
The process of renumbering is straightforward in theory but can be
difficult and dangerous in practice. The threats fall into two broad
categories: those arising from misconfiguration and those that are
actual attacks.
Misconfigurations can easily arise if any system in the network
"knows" the old prefix, or an address in it, a priori and is not
configured with the new prefix, or if the new prefix is configured in
a manner that replaces the old instead of being co-equal to it for a
period of time. Simplistic examples include the following:
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Neglecting to reconfigure a system that is using the old prefix in
some static configuration: in this case, when the old prefix is
removed from the network, whatever feature was so configured
becomes inoperative - it is not configured for the new prefix, and
the old prefix is irrelevant.
Configuring a system via an IPv6 address, and replacing that old
address with a new address: because the TCP connection is using
the old and now invalid IPv6 address, the SSH session will be
terminated and you will have to use SSH through the new address
for additional configuration changes.
Removing the old configuration before supplying the new: in this
case, it may be necessary to obtain on-site support or travel to
the system and access it via its console.
Clearly, taking the extra time to add the new prefix to the
configuration, allowing the network to settle, and then removing the
old obviates this class of issue. A special consideration applies
when some devices are only occasionally used; the administration must
allow a sufficient length of time in Section 2.6 or apply other
verification procedures to ensure that their likelihood of detection
is sufficiently high.
A subtle case of this type can result when the DNS is used to
populate access control lists and similar security or QoS
configurations. DNS names used to translate between system or
service names and corresponding addresses are treated in this
procedure as providing the address in the preferred prefix, which is
either the old or new prefix but not both. Such DNS names provide a
means, as described in Section 2.6, to cause systems in the network
to stop using the old prefix to access servers or peers and cause
them to start using the new prefix. DNS names used for access
control lists, however, need to go through the same three-step
procedure used for other access control lists, having the new prefix
added to them as discussed in Section 2.3 and the old prefix removed
as discussed in Section 2.7.
It should be noted that the use of DNS names in this way is not
universally accepted as a solution to this problem; [RFC3871]
especially notes cases where static IP addresses are preferred over
DNS names, in order to avoid a name lookup when the naming system is
inaccessible or when the result of the lookup may be one of several
interfaces or systems. In such cases, extra care must be taken to
manage renumbering properly.
Attacks are also possible. Suppose, for example, that the new prefix
has been presented by a service provider, and the service provider
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starts advertising the prefix before the customer network is ready.
The new prefix might be targeted in a distributed denial of service
attack, or a system might be broken into using an application that
would not cross the firewall using the old prefix, before the
network's defenses have been configured. Clearly, one wants to
configure the defenses first and only then accessibility and routing,
as described in Section 2.3 and Section 3.3.
The SLAC procedure described in [RFC2462] renumbers hosts. Dynamic
DNS provides a capability for updating DNS accordingly. Managing
configuration items apart from those procedures is most obviously
straightforward if all such configurations are generated from a
central configuration repository or database, or if they can all be
read into a temporary database, changed using appropriate scripts,
and applied to the appropriate systems. Any place where scripted
configuration management is not possible or is not used must be
tracked and managed manually. Here, there be dragons.
In ingress filtering of a multihomed network, an easy solution to the
issues raised in Section 3.3 might recommend that ingress filtering
should not be done for multihomed customers or that ingress filtering
should be special-cased. However, this has an impact on Internet
security. A sufficient level of ingress filtering is needed to
prevent attacks using spoofed source addresses. Another problem
comes from the fact that if ingress filtering is made too difficult
(e.g., by requiring special-casing in every ISP doing it), it might
not be done at an ISP at all. Therefore, any mechanism depending on
relaxing ingress filtering checks should be dealt with with extreme
care.
6.Acknowledgements
This document grew out of a discussion on the IETF list. Commentary
on the document came from Bill Fenner, Christian Huitema, Craig
Huegen, Dan Wing, Fred Templin, Hans Kruse, Harald Tveit Alvestrand,
Iljitsch van Beijnum, Jeff Wells, John Schnizlein, Laurent Nicolas,
Michael Thomas, Michel Py, Ole Troan, Pekka Savola, Peter Elford,
Roland Dobbins, Scott Bradner, Sean Convery, and Tony Hain.
Some took it on themselves to convince the authors that the concept
of network renumbering as a normal or frequent procedure is daft.
Their comments, if they result in improved address management
practices in networks, may be the best contribution this note has to
offer.
Christian Huitema, Pekka Savola, and Iljitsch van Beijnum described
the ingress filtering issues. These made their way separately into
[RFC3704], which should be read and understood by anyone who will
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Appendix A. Managing Latency in the DNS
The procedure in this section can be used to determine and manage the
latency in updates to information a DNS resource record (RR).
There are several kinds of possible delays that are ignored in these
calculations:
o the time it takes for the administrators to make the changes;
o the time it may take to wait for the DNS update, if the
secondaries are only updated at regular intervals, and not
immediately; and
o the time the updating to all the secondaries takes.
Assume the use of NOTIFY [RFC1996] and IXFR [RFC1995] to transfer
updated information from the primary DNS server to any secondary
servers; this is a very quick update process, and the actual time to
update of information is not considered significant.
There is a target time, TC, at which we want to change the contents
of a DNS RR. The RR is currently configured with TTL == TTLOLD. Any
cached references to the RR will expire no more than TTLOLD in the
future.
At time TC - (TTLOLD + TTLNEW), the RR in the primary is configured
with TTLNEW (TTLNEW < TTLOLD). The update process is initiated to
push the RR to the secondaries. After the update, responses to
queries for the RR are returned with TTLNEW. There are still some
cached references with TTLOLD.
At time TC - TTLNEW, the RR in the primary is configured with the new
address. The update process is initiated to push the RR to the
secondaries. After the update, responses to queries for the RR
return the new address. All the cached references have TTLNEW.
Between this time and TC, responses to queries for the RR may be
returned with either the old address or the new address. This
ambiguity is acceptable, assuming the host is configured to respond
to both addresses.
At time TC, all the cached references with the old address have
expired, and all subsequent queries will return the new address.
After TC (corresponding to the final state described in Section 2.8),
the TTL on the RR can be set to the initial value TTLOLD.
The network administrator can choose TTLOLD and TTLNEW to meet local
requirements.
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As a concrete example, consider a case where TTLOLD is a week (168
hours) and TTLNEW is an hour. The preparation for the change of
addresses begins 169 hours before the address change. After 168
hours have passed and only one hour is left, the TTLNEW has
propagated everywhere, and one can change the address record(s).
These are propagated within the hour, after which one can restore TTL
value to a larger value. This approach minimizes time where it is
uncertain what kind of (address) information is returned from the
DNS.
Authors' Addresses
Fred Baker
Cisco Systems
1121 Via Del Rey
Santa Barbara, CA 93117
US
Phone: 408-526-4257
Fax: 413-473-2403
EMail: fred@cisco.com
Eliot Lear
Cisco Systems GmbH
Glatt-com 2nd Floor
CH-8301 Glattzentrum
Switzerland
Phone: +41 1 878 9200
EMail: lear@cisco.com
Ralph Droms
Cisco Systems
200 Beaver Brook Road
Boxborough, MA 01719
US
Phone: +1 978 936-1674
EMail: rdroms@cisco.com
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Full Copyright Statement
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